High-resolution MERLIN maps are presented of the OH and H2O masers in Cepheus A. These masers occur in four well-defined groups associated with four of the ultracompact H II regions in Cep A. From the present measurements it appears that the most compact of the H II regions is the source of the large-scale bipolar outflow which has been observed in CO, and that the outflow is collimated within ∼ 1015 cm of the central young star. The bipolar outflow is aligned in position angle with the interstellar magnetic-field lines, and the collapsing molecular cloud is oriented at right angles to the field lines. A similar configuration is found in many other bipolar sources. This suggests that the interstellar magnetic field has played an important role in the evolution of these sources.